1. Technical Field
The present invention relates to a wafer, and more particularly to a wafer having a common spare through silicon via (TSV) and a method of improving a yield rate thereof.
2. Background
In a through silicon via (TSV) technique, a hole is first drilled on a wafer by etching or laser, then a conductive material is filled into the hole to connect a circuit. Next, the wafer or a die is thinned for stacking and bonding. The TSV technique is adopted as a stacking technique for signal transmission between wafers.
FIG. 1 is a schematic diagram illustrating a TSV of a conventional wafer. Referring to FIG. 1, in a cross-sectional view, a circuit unit 101 inside a wafer 100 is coupled to a TSV 102 and a pad 105 via a front metal FM of the wafer 100. Moreover, the circuit unit 101 is also coupled to a pad 104 via the front metal FM of the wafer 100, the TSV 102, and a back metal BM of the wafer 100. The circuit unit 101 is capable of transmitting signals with circuit units (not shown) inside wafers of upper and lower layers via the TSV 102 and the pads 104 and 105. Therefore, a die stack is achieved using the TSV technique.
However, the die stack using the TSV technique for three-dimensional integration has a higher throughput in a wafer-to-wafer process than other processes in a bonding method. A yield of the die stack not only depends on a quality of the wafer itself, but is also closely related to the TSV technique. Although the yield product of the die stack can be enhanced by operating various techniques, such as improving system defects or selecting known good dies (KGDs) with smaller location differences, the last obstacle still regards to the yield of the TSV technique.
Comparing to through holes connecting every metal layer inside the wafer, the TSV needs to penetrate a substrate of the wafer, thus has a deeper depth and a higher failure rate.
In addition, a function of the TSV is generally categorized into four types: signal transmission, power delivery, thermal conduction, and input/output port connection. Inherent design demands of the latter three types include an application of a plurality of (more than one) bonds or a preference of adopting a TSV with a greater diameter, thus have small effects on the yield and reliability. However, the TSV for signal transmission does not have the same design demands. The TSV for signal transmission have higher interconnection density, so the yield of the die stack is easily affected by the yield thereof. Hence, the yield needs to be improved by laying out spare TSVs.
It should be noted that the diameter of the TSV ranges from few micrometers to tens of micrometers. Comparing to the semiconductor process technique with a nanometer scale, the diameter of the TSV is obviously much greater. Since failures happen randomly, it is necessary for every normal TSV to have a spare. As a result, the yield is increased and the reliability is enhanced, but an area of the wafer is increased as well. Nevertheless, in the numerous TSVs, only a few would fail and require repairment. When the spare TSVs are not being used, they are redundancies that occupy the wafer area. If only a portion of the TSVs are provided with the spare TSVs, an overall yield is still affected by the failure rate of a portion of the TSVs not provided with the spare TSVs.
FIG. 2 is a schematic diagram illustrating a TSV of another conventional wafer with a spare TSV 203. Referring to FIG. 2, a wafer 200 applies a one-to-one sparing manner, so that the spare TSV 203 is added beside each normal TSV 202. A circuit unit 201 is coupled to the TSV 202 and the spare TSV 203 via a front metal FM of the wafer 200. Besides, the circuit unit 201 is also coupled to a back metal BM of the wafer 200 via the front metal FM of the wafer 200, the TSV 202, and the spare TSV 203.